A tungsten alloy part which utilizes the high-temperature strength of tungsten is used in various fields. Examples thereof include a discharge lamp, a transmitting tube, and a magnetron. The tungsten alloy part is used for a cathode electrode, an electrode supporting rod, and a coil part or the like in the discharge lamp (HID lamp). The tungsten alloy part is used for a filament and a mesh grid or the like in the transmitting tube. The tungsten alloy part is used for the coil part or the like in the magnetron. These tungsten alloy parts include a sintered body having a predetermined shape, a wire rod, and a coil part obtained by processing the wire rod into a coil form.
Conventionally, as described in Jpn. Pat. Appln, KOKAI Publication No. 2002-226935 (Patent Literature 1), a tungsten alloy containing thorium (or a thorium compound) is used for these tungsten alloy parts. In the tungsten alloy of Patent Literature 1, deformation resistance is improved by finely dispersing thorium particles and thorium compound particles so that the average particle diameter thereof is set to 0.3 μm or less. Since the thorium-containing tungsten alloy has excellent emitter characteristics and mechanical strength at a high temperature, the thorium-containing tungsten alloy is used in the above fields.
However, since thorium or the thorium compound is a radioactive material, a tungsten alloy part using no thorium is desired in consideration of the influence on the environment. In Jpn. Pat. Appln. KOKAI Publication No. 2011-103240 (Patent Literature 2), a tungsten alloy part containing boride lanthanum (LaB6) has been developed as the tungsten alloy part using no thorium.
On the other hand, a short arc type high-pressure discharge lamp using a tungsten alloy containing lanthanum trioxide (La2O3) and HfO2 or ZrO2 is described in Patent Literature 3. According to the tungsten alloy described in Patent Literature 3, sufficient emission characteristics are not obtained. This is because lanthanum trioxide has a low melting point of about 2300° C., and lanthanum trioxide is evaporated in an early stage when a part is subjected to a high temperature by increasing an applied voltage or a current density, which causes deterioration in emission characteristics.